1. Field of the Invention
The present invention relates generally to surface-pressure and shear-tension measurements and, more particularly, relates to surface-pressure and shear-tension measurements based on the deformation of an elastic film located in association with the surface of an object or model.
2. Description of Prior Art
Convenient, reliable and inexpensive methods for determining pressure maps of surfaces, particularly on aerodynamic objects, are currently under development. One known approach is to employ pressure taps which are drilled into the surface under study and connected via tubing to multiplexed pressure gages or pressure gage arrays. Hundreds of pressure taps are generally required to get a pressure distribution over an entire surface. Even then, the significant distance between taps usually requires additional sophisticated interpolation procedures. Pressure taps provide information only about the static or normal pressure component acting on the surface. Tangential pressure components or shear stresses should also be measured to fully characterize the forces acting on the surface. Up to now these components have been measured mostly by indirect approaches such as heat flux measurements, by miniature balance systems imbedded into the object or model surface, or by measuring the shear force acting on a floated piece of the model surface.
An additional technique proposed utilizing the oxygen quenching of fluorescent dyes for flow visualization. Since then a substantial effort has gone into the development of the Pressure Sensitive Paint (PSP) technique which is a quantitative surface pressure measurement method. The PSP technique is based on the oxygen quenching of molecular photoluminescence. A surface under study is covered by an oxygen-permeable thin polymeric layer containing an embedded oxygen sensitive luminophore. Oxygen from the ambient flow can diffuse into the polymeric layer where its concentration is proportional to the static air pressure. Luminescence from the embedded probe molecules is excited with the appropriate light source. Due to the quenching effect of oxygen on molecular photoluminescence, the luminescence intensity from the coated surface is an indicator of the local oxygen concentration and thus represents a method to obtain surface pressure. Oxygen diffusion in the PSP layer determines its response to pressure changes on the surface. The response time can be estimated as τ≈Λ2/D, where Λ is the PSP layer thickness and D is the oxygen diffusion coefficient. For a practical PSP layer thickness of 1–2microns, the response time is between 2–4 ms. Such a slow response time represents a significant barrier to utilizing PSPs for the investigation of transient fluid phenomena.
The PSP technique based on oxygen quenching is essentially an absolute pressure gage. The sensitivity, dIr/dP, for most currently available PSP formulations generally vary in the range of 0.0005 to 0.001%/Pa. Unfortunately, this sensitivity can not be increased significantly due to both the physical and photochemical properties of available formulations. Thus, it is very difficult to utilize PSP in applications with small pressure variations such as measurements at low subsonic velocities (Mach number (Ma) below 0.05). Pressure variation on a model is proportional to the square of the Mach number, thus, the pressure variation range, ΔP, for a flow at Ma=0.05 (V=35 mile/h) is approximately 100 Pa which is substantially less than the pressure variation range, ΔP, for a flow at M=0.5 (V=350 mile/h and ΔP=104 Pa). Since the PSP sensitivity can not be modified to accurately cover this low scale, reliable results can generally only be obtained by increasing the signal to noise ratio of the acquired intensity (typically by averaging many images) and compensating for all possible error sources such as illumination non-stability, model displacement and deformation, and temperature effects. PSP temperature sensitivity varies between 100 Pa/° C. to 1000 Pa/° C. and is comparable with the total pressure variation range for a flow at Ma=0.05. Thus, PSP measurements at flow velocities below Ma=0.05 require significant efforts on the part of the user to obtain quality data.
Mapping of shear stresses has generally been accomplished in the past by using liquid crystals or oil film measurements. A further method of shear stress mapping involved mounting on a model surface a sensing element in the form of a film made of a flexible polymer gel of a small thickness with a known shear modulus. Markers applied to the film and the model surfaces were used to record the shear deformation of the film under aerodynamic loading. Shearing stress in this case is determined using Hooke's law for shear strain. The markers can be in the form of a grating placed both on the model's surface (under the film) and on the film's surface enabling the use of the Moiré technique for recording shear strain. A drawback of this method is the fact that gradients of the normal pressure can also create a shear displacement of the polymer gel and thus this method works best in the absence of normal pressure gradients. On the other hand, the normal film deformation or displacement is not very sensitive to shear force action. Practical flow investigations are characterized by regions with large pressure spikes and pressure gradients, but usually display shear forces and shear gradients that are 10 to 1000 times smaller. This creates a difficult problem in determining shear deformation from normal and shear surface load components.